1. Field of the Invention
The present invention relates to a reactive, silane-based thixotropic hot-melt adhesive.
Traditionally, hot-melt adhesives (hot melts) are thermoplastic materials, which are heated for their processing in a dispensing gun or in another device to a temperature of 90.degree.-200.degree. C. and are applied as a liquid mass to the substrates to be bonded.
During cooling, the mass solidifies and thus quickly forms a stable bond.
Quick solidification and the absence of water and solvents are the advantages of these hot-melt adhesives, compared with other adhesive systems.
But their thermoplastic behavior is a drawback, because their mechanical and therefore adhesive characteristics are strongly dependent on temperature. At elevated temperature they re-soften, and at lower temperature they tend to become brittle. Thus, they may only be used in a relatively narrow temperature range.
In the last few years, reactive polyurethane hot-melt adhesives have been introduced in the market. They are characterized by an ability to become cross-linked, whereupon they lose their thermoplastic behavior and have good mechanical characteristics over a broad temperature range. Cross-linkage of the reactive isocyanate groups can be achieved with water, especially humidity, or with a hydrogen-activity compound that is liberated by the influence of heat.
The known solvent-free, moisture-hardenable prepolymeric isocyanates are liquid to soft-waxlike at room temperature. Moreover, because of their low green strength and long reaction time, they have only limited applications.
2. Description of the Prior Art
DE-PS 2 401 320 discloses isocyanate-containing hot-melt adhesives, which are composed of:
(a) 20 to 73 percent by weight of a urethane prepolymer having terminal isocyanate groups on both ends;
(b) 2 to 25 percent by weight of a thermoplastic polymer selected from the group consisting of ethylene-vinylacetate copolymers, ethylene-acrylic acid copolymers, ethylene-acrylate copolymers, atactic polypropylene and linear polyethyleneterephthalate polymers; and
(c) 25 to 55 percent by weight of a tackifying resin, such as colophonium derivatives or terpene-phenol copolymers.
However, at certain mixing ratios of these components, they tend to be incompatible and cause instability of the molten masses. Also, they have an insufficient resistance to continuous stress. Moreover, their storage stability is sometimes not sufficient.
In DE-OS 3 236 313 there are described reactive hot-melt adhesives which are compatible, stable upon melting, storage stable in the absence of moisture, and which adhere strongly from the outset and have high thermostability. These reactive hot-melt adhesives were developed for the bonding of thermoplastic and duroplastic materials, foam plastics, varnished surfaces, wood, paper, leather, rubber, textiles, metals, etc. They consist of a mixture of:
(a) 20 to 90 percent by weight of a prepolymeric isocyanate,
(b) 0 to 75 percent by weight of a thermoplastic polyurethane, and
(c) 5 to 50 percent by weight of a low molecular weight synthetic resin selected from the group consisting of ketone resins, ketone-aldehyde condensation resins, and hydrogenation products of acetophenone condensation resins.
These reactive hot-melt adhesives, within the ranges set forth above, have only limited sag resistance during coating (a coating thickness of more than 2 mm may not be guaranteed), are very tacky and string-like, and after cooling are viscous and have poor plastic moldability. Therefore, their use is limited to certain applications.
There shortcomings of the art are overcome by the subject matter disclosed in Swiss patent application no. 1902/86-4, comprising reactive hot-melt adhesives which do not sag during application, and which, after cooling to room temperature, are touchable and plastically moldable. After cross-linkage with water, especially moisture, these adhesives have good mechanical characteristics and are rubbery to viscous elastic independent of the application temperature, i.e., the melting temperature or ambient temperature. Due to their stability or to their resistance to continuous stress during application, many new applications are possible. A particularly promising new application of these adhesives is in the carmaking industry, where they serve as adhesive bondings for windshields, adhesive bondings for plastic strips, etc. Also, they serve as adhesive bondings in the production of marine vessels, e.g., adhesive bondings for wooden parts.
Swiss patent application no. 1902/86-4 also provides a process for the preparation of these reactive hot-melt adhesives. The reactive hot-melt adhesive disclosed therein is preferably viscous, plastic and moldable at room temperature, and contains a mixture of:
at least one thermoplastic material, preferably in an amount from 20 to 90 percent by weight, especially from 50 to 65 percent by weight, PA1 at least one binder having reactive isocyanate groups, preferably in an amount from 5 to 40 percent by weight, especially from 15 to 30 percent by weight, and PA1 at least one thixotropic agent, preferably in an amount from 3 to 30 percent by weight, especially from 8 to 20 percent by weight,
wherein the isocyanate content, relative to the total weight of the hot-melt adhesive, is in the range from 0.01 to 10 percent by weight, preferably in the range from 0.1 to 1 percent by weight, and especially in the range from 0.3 to 0.7 percent by weight.